This research proposal has several primary objectives: 1. A detailed analysis of the emerging oral facial region of the mammalian (mouse) embryo. 2. Ultrastructural analysis of the interrelationship among the progenitor tissues from which the oral facial region arises. 3. Experimental in vitro analysis of the interactions among these progenitor tissues of the early embryo. 4. Experimental in vitro analysis of the tissue interactions "responsible" for development and differentiation of both progenitor tissues and, in later embryos, of tissues comprising the mandibular arch.